1. Field of the Invention
The present invention relates to a semiconductor laser device made of a group III-V nitride based semiconductor (hereinafter referred to as a nitride based semiconductor) such as BN (boron nitride), GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride) or TIN (thallium nitride) or mixed crystals thereof.
2. Description of the Prior Art
In recent years, GaN based semiconductor light emitting devices are increasingly put into practice as semiconductor laser devices emitting blue or violet light. Such semiconductor laser devices are generally employed as the light sources for optical disk systems writing and reading information in and from an optical disk among optical memories optically writing or reading information. In particular, GaN based semiconductor laser devices are expected as the light sources for high-density optical disk systems such as advanced digital video disks.
FIG. 12 is a typical sectional view of a conventional GaN based semiconductor laser device. Referring to FIG. 12, an undoped GaN low-temperature buffer layer 52, an undoped GaN layer 53, an n-GaN layer 54, an n-anti-cracking layer 55, an n-AlGaN cladding layer 56, an n-GaN light guide layer 57 and an InGaN multiple quantum well (MQW) active layer 58 are successively provided on a sapphire substrate 51.
Further, a p-AlGaN carrier blocking layer 59, a p-GaN light guide layer 60 and a p-AlGaN first cladding layer 61 are successively provided on the active layer 58. An n-GaN current blocking layer 62 having a striped opening 63 is formed on the p-AlGaN first cladding layer 61. A p-AlGaN second cladding layer 64 and a p-GaN contact layer 65 are successively provided on the p-AlGaN first cladding layer 61 located in the striped opening 63 and the n-GaN current blocking layer 62.
Partial regions of the layers from the p-GaN contact layer 65 to the n-GaN layer 54 are removed by etching, to expose the n-GaN layer 54. A p type electrode 66 is formed on the upper surface of the p-GaN contact layer 65, and an n type electrode 67 is formed on the exposed upper surface of the n-GaN layer 54.
In the semiconductor laser device shown in FIG. 12, electrons (negative carriers) supplied from the n type electrode 67 are injected into the active layer 58 through the n-GaN layer 54, the n-anti-cracking layer 55, the n-AlGaN cladding layer 56 and the n-GaN light guide layer 57. Holes (positive carriers) supplied from the p type electrode 66 are injected into the active layer 58 through the p-GaN contact layer 65, the p-AlGaN second cladding layer 64, the p-AlGaN first cladding layer 61, the p-GaN light guide layer 60 and the p-AlGaN carrier blocking layer 59.
The n-GaN current blocking layer 62 having the striped opening 63 is provided in order to reduce operating current and limit an emission spot position by limiting the flow of current in a striped manner. The n-GaN current blocking layer 62 limits the current flowing into the active layer 58 substantially to the region located under the striped opening 63.
In the aforementioned conventional semiconductor laser device, however, the n-GaN current blocking layer 62, having a larger refractive index as compared with the p-AlGaN first cladding layer 61 and the p-AlGaN second cladding layer 64, has no effect of light confinement.
In order to bring the semiconductor laser device into a real refractive index guided structure for attaining the effect of light confinement, the n-current blocking layer must be made of n-AlGaN, for example, in a larger Al composition ratio than the p-AlGaN first cladding layer 61 and the p-AlGaN second cladding layer 64 so that the refractive index thereof is smaller than those of the p-AlGaN first cladding layer 61 and the p-AlGaN second cladding layer 64. Thus, the effective refractive index in the region of the active layer 58 located under the n-current blocking layer is smaller than that of the region of the active layer 58 located under the p-AlGaN second cladding layer 64 in the striped opening 63. Consequently, light is confined in the central portion of the active layer 58. In this case, however, the p-AlGaN first cladding layer 61 and the p-AlGaN second cladding layer 64, which are also made of n-AlGaN, and the n-AlGaN current blocking layer provide a large AlGaN film thickness in total. Such a film having a large Al composition is readily cracked if the thickness thereof is too large.
An object of the present invention is to provide a semiconductor laser device having a current blocking layer, which is excellent in thermal stability and prevented from cracking.
Another object of the present invention is to provide a semiconductor laser device having a current blocking layer, which is excellent in thermal stability, prevented from cracking and improved in effect of light confinement.
A semiconductor laser device according to an aspect of the present invention comprises a first nitride based semiconductor layer including an active layer and containing at least one of boron, aluminum, gallium, indium and thallium, a current blocking layer, formed on the first nitride based semiconductor layer, having a striped opening, and a second nitride based semiconductor layer, formed on the first nitride based semiconductor layer in the striped opening, containing at least one of boron, aluminum, gallium, indium and thallium, and the current blocking layer includes a multilayer structure of at least one first layer of a nitride based semiconductor containing at least one of aluminum and boron and at least one second layer of a nitride based semiconductor containing indium and having a smaller band gap than the first layer.
In this semiconductor laser device, the first layer of the current blocking layer contains at least one of boron and aluminum, whereby the band gap of the first layer can be increased for reducing the refractive index of the first layer. Thus, in the case of a real refractive index guided structure, the difference in refractive index between the current blocking layer and the second nitride based semiconductor layer in the striped opening can be increased. Further, the first layer of the current blocking layer containing at least one of boron and aluminum is thermally stabilized. In addition, the second layer of the current blocking layer containing indium can absorb strain caused in the first layer containing boron or aluminum. Thus, cracking is suppressed.
Therefore, a semiconductor laser device having a real refractive index guided structure excellent in thermal stability, prevented from cracking and improved in effect of light confinement is implemented. Alternatively, a semiconductor laser device having a loss guided structure excellent in thermal stability and prevented from cracking is implemented.
At least one first layer may have a larger aluminum composition ratio than that of at least one second layer or a larger boron composition ratio than that of at least one second layer, and at least one second layer may have a larger indium composition ratio than that of at least one first layer.
In this case, at least one first layer is reduced in refractive index and improved in thermal stability. Further, at least one second layer absorbs strain of at least one first layer.
The first nitride based semiconductor layer may include a first conductivity type cladding layer, the active layer and a second conductivity type first cladding layer in this order, and the second nitride based semiconductor layer may include a second conductivity type second cladding layer.
At least one first layer may include at least two first layers, and at least two first layers and at least one second layer may be alternately stacked.
In this case, the second layer held between the first layers effectively absorbs strain caused in the first layers arranged on both sides thereof.
The mean refractive index of the current blocking layer may be smaller than the refractive index of the second nitride based semiconductor layer in the striped opening. In this case, the effective refractive index in the region of the active layer located under the current blocking layer is smaller than the effective refractive index in the region of the active layer located under the striped opening, and light is concentrated to the region of the active layer located under the striped opening. Thus, a semiconductor laser device having a real refractive index guided structure is implemented.
The mean band gap of the current blocking layer may be larger than the band gap of the second conductivity type second cladding layer. In this case, the mean refractive index of the current blocking layer is smaller than the refractive index of the second conductivity type second cladding layer in the striped opening. Thus, the effective refractive index in the region of the active layer located under the current blocking layer is smaller than the effective refractive index in the region of the active layer located under the second conductivity type second cladding layer in the striped opening, and light is concentrated to the region of the active layer located under the striped opening. Consequently, a semiconductor laser device having a real refractive index guided structure is implemented.
The active layer may include at least one quantum well layer and at least two quantum barrier layers alternately stacked, and the band gap of at least one second layer may be larger than the band gap of at least one quantum well layer. In this case, the mean band gap of the current blocking layer can be readily rendered larger than the band gap of the second conductivity type second cladding layer.
The active layer may contain indium, gallium and nitrogen, at least one first layer may contain at least one of aluminum and boron as well as gallium and nitrogen, at least one second layer may contain indium, gallium and nitrogen, and the first conductivity type cladding layer, the second conductivity type first cladding layer and the second conductivity type second cladding layer may contain aluminum, gallium and nitrogen.
At least one first layer may have a larger aluminum composition ratio than that of the second conductivity type second cladding layer or a larger boron composition ratio than that of the second conductivity type second cladding layer. Thus, the band gap of at least one first layer is larger than the band gap of the second conductivity type second cladding layer.
The active layer may include at least one quantum well layer containing indium, gallium and nitrogen and at least two quantum barrier layers containing indium, gallium and nitrogen, alternately stacked, and the indium composition ratio of at least one quantum well layer may be larger than the indium composition ratio of at least two quantum barrier layers.
The mean band gap of the current blocking layer may be substantially equal to or smaller than the band gap of the active layer.
In this case, the current blocking layer absorbs light generated in the region of the active layer located under the current blocking layer, whereby light is concentrated to the region of the active layer located under the striped opening. Thus, a semiconductor laser device having a loss guided structure is implemented.
The active layer may include at least one quantum well layer and at least two quantum barrier layers alternately stacked, and the mean band gap of the current blocking layer may be substantially equal to or smaller than the mean band gap of the active layer. In this case, the current blocking layer absorbs light generated in the active layer having a multiple quantum well structure, whereby light is concentrated to the region of the active layer located under the striped opening.
The band gap of at least one second layer may be smaller than the band gap of at least one quantum well layer. In this case, the mean band gap of the current blocking layer can be readily rendered substantially equal to or smaller than the band gap of the active layer.
The active layer may contain indium, gallium and nitrogen, at least one first layer may contain at least one of aluminum and boron as well as indium, gallium and nitrogen, at least one second layer may contain indium, gallium and nitrogen, and the first conductivity type cladding layer, the second conductivity type first cladding layer and the second conductivity type second cladding layer may contain aluminum, gallium and nitrogen.
The active layer may include at least one quantum well layer containing indium, gallium and nitrogen and at least two quantum barrier layers containing indium, gallium and nitrogen, alternately stacked, and the indium composition ratio of at least one quantum well layer may be larger than the indium composition ratio of at least two quantum barrier layers.
The first nitride based semiconductor layer may further include a first conductivity type light guide layer provided between the first conductivity type cladding layer and the active layer and a second conductivity type light guide layer provided between the active layer and the second conductivity type first cladding layer.
The semiconductor laser device may further comprise a third nitride based semiconductor layer, formed on the current blocking layer and the second nitride based semiconductor layer, containing at least one of boron, aluminum, gallium, indium and thallium.
The third nitride based semiconductor layer may include a third cladding layer of the second conductivity type and a second conductivity type contact layer. Alternatively, the third nitride based semiconductor layer may include a second conductivity type contact layer.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.